The atmospheric visibility that is available to operators of various types of vehicles during travel is an important safety concern. Numerous accidents can be attributed to travel of vehicle during low visibility. At night, it can be especially difficult to determine the visibility conditions. Low visibility raises risks leading to spatial disorientation, threat of hitting obstacles, and mid-air collisions.
For aircraft, Federal Aviation Administration (FAA) flight rules require flight visibility minimums for certain types of flying, for example Visual Flight Rules (VFR), Instrument Flight Rules (IFR), and instrument approaches. Determining in flight visibility is a requirement of pilots, both under VFR and IFR. Maintaining minimum visibility is a critical concept in FAA regulations for VFR to help avoid colliding with other aircraft, as well as for avoiding mountains, towers, or any other object. Another major factor in low visibility is spatial disorientation of pilots, which can occur in hazy, low visibility situations that obscure the horizon.
Currently, flight visibility is determined by pilot estimates based on visual reference to ground objects assumed to be a known distance from the moving aircraft. This approach can be very inaccurate based on conditions and is not always possible (e.g., during night flight). There are no sensors or equipment that can detect and report in-flight visibility in all phases of flight. Airports have equipment on the ground that can provide ground visibility reporting, but those values are limited to the location of the equipment and do not satisfy the requirements for the pilot in determining the flight visibility. Unfortunately, depending on the conditions, the visibility estimates by pilots can lead to both legal and safety concerns.
For example, Federal Aviation Regulation 91.175 requires that no pilot may operate an aircraft below the authorized minimum descent altitude or continue an approach below the authorized descent altitude/height unless the flight visibility is not less than the visibility prescribed in the standard instrument approach being used. In the past, this has caused FAA legal action against pilots when the reported airport ground visibility is lower than the visibility for the approach, and the pilot has to argue the case that the flight visibility was at or above the minimum without any verifiable evidence.